Re: BUG #19593: area(circle) silently returns Infinity instead of raising "value out of range: overflow"

Hi, Michael!

Agreed — PostgreSQL's check is fine, this is a gcc wrong-code bug.

circle_ar() is just two inlined float8_mul() calls:
```
return float8_mul(float8_mul(circle->radius, circle->radius), M_PI);
```
For radius 1e154, r*r is still finite (~1e308), but (r*r)*pi overflows.
float8_mul() is supposed to catch that with:
```
result = val1 * val2;
if (unlikely(isinf(result)) && !isinf(val1) && !isinf(val2))
float_overflow_error();
```
That check is present in the source and survives at -O0, with clang,
and for the equivalent SQL expression
1e154::float8 * 1e154::float8 * pi() (as in the report). With gcc -O1
and above it is deleted for the outer multiply.

What gcc does is jump threading (-fthread-jumps; enabled by default at
-O1 and above). After the inner check it knows r*r is finite, M_PI is
a finite constant, and it incorrectly treats "finite * finite yields
Inf" as impossible. The outer float_overflow_error() call disappears;
the function just returns Infinity. -O2 -fno-thread-jumps restores the
correct behaviour. This is with a normal -O2 build; we do not use
-ffinite-math-only / -ffast-math, so gcc is not entitled to assume that
finite*finite cannot produce Infinity.

Reduced C reproducer (same shape as circle_ar / float8_mul). On the
correct path it prints the same message as PostgreSQL:
```
ERROR: value out of range: overflow
```
On the buggy path it prints Infinity (and exits 1):
```
Host: Linux x86_64
gcc: gcc (Ubuntu 15.2.0-16ubuntu1) 15.2.0
clang: Ubuntu clang version 21.1.8

compiler / flags result
------------------------------- ------------------------------------
gcc -O0 OK (ERROR: value out of range...)
gcc -O1 BUG (Infinity)
gcc -O2 BUG (Infinity)
gcc -O2 -fno-thread-jumps OK (ERROR: value out of range...)
clang -O0 OK (ERROR: value out of range...)
clang -O1 OK (ERROR: value out of range...)
clang -O2 OK (ERROR: value out of range...)
```
So this is not "PostgreSQL does not check circle areas". The check is
there, gcc optimizes it away, clang does not. The same pattern can
affect other inlined float4/8 helpers when both operands are proven
finite.

One possible workaround on our side would be to store the operation
result in a volatile temporary in those helpers, so the isinf() check
must inspect the computed value. But it seems to me that we should
address the root cause first: file this with gcc bugzilla and decide
what, if anything, to do in PostgreSQL based on their response.

сб, 1 авг. 2026 г. в 18:02, PG Bug reporting form <noreply(at)postgresql(dot)org>:

> The following bug has been logged on the website:
>
> Bug reference: 19593
> Logged by: Michael Malis
> Email address: malis(at)pgrust(dot)com
> PostgreSQL version: 18.3
> Operating system: Debian (official Docker image), aarch64
> Description:
>
> I'm not sure what you'll want to do with this one, but I figured I would at
> least report it. The cause seems to be a bug in gcc.
>
> area(circle) returns Infinity where it must raise ERROR 22003 "value out of
> range: overflow". The overflow check in float8_mul() is present in the
> source
> but is absent from the generated code, because gcc 13 and later delete it
> at
> -O1 and above.
>
> The same binary raises the error correctly for the equivalent SQL-level
> expression, and for a circle whose radius overflows one step earlier, so
> this
> is not "PostgreSQL does not check circle areas".
>
> -- WRONG: no error, returns Infinity
> SELECT area(circle '<(0,0),1e154>');
> area
> ------------------------
> Infinity
>
> -- CORRECT (control): the *inner* multiply overflows, so the surviving
> -- check fires
> SELECT area(circle '<(0,0),1e200>');
> ERROR: value out of range: overflow
>
> -- CORRECT (control): the same arithmetic, expressed in SQL
> SELECT 1e154::float8 * 1e154::float8 * pi();
> ERROR: value out of range: overflow
>
> -- sane value, for reference
> SELECT area(circle '<(0,0),1e10>');
> area
> -------------------------
> 3.1415926535897933e+20
>
> 1e154 * 1e154 = 1e308, which is finite (below DBL_MAX); multiplying that by
> pi
> overflows. Behaviour is identical whether the expression is
> constant-folded
> at
> plan time or evaluated at runtime:
>
> SELECT area(c) FROM (VALUES (circle '<(0,0),1e154>')) t(c); --
> Infinity
>
> NaN and Infinity radii behave correctly (NaN -> NaN, Infinity -> Infinity).
>
>
> WHERE IT COMES FROM
>
> src/backend/utils/adt/geo_ops.c:5159
>
> static float8
> circle_ar(CIRCLE *circle)
> {
> return float8_mul(float8_mul(circle->radius, circle->radius),
> M_PI);
> }
>
> src/include/utils/float.h:207
>
> static inline float8
> float8_mul(const float8 val1, const float8 val2)
> {
> float8 result;
>
> result = val1 * val2;
> if (unlikely(isinf(result)) && !isinf(val1) && !isinf(val2))
> float_overflow_error();
> ...
>
> Two float8_mul calls are inlined into one function. gcc keeps the first
> copy's
> overflow check and deletes the second's.
>
> Disassembly of the shipped binary (circle_area; symbols are present in
> .dynsym).
> gcc lowers isinf(x) to |x| > DBL_MAX, with d30 = 0x7fefffffffffffff:
>
> ; inner multiply -- check intact
> 5540dc fmul d31, d29, d29 ; r*r
> 5540e0 fcmp d31, d30
> 5540e4 b.le 5540fc
> 5540e8 fabs d29, d29 ; |r| <- the !isinf(val1) test
> 5540ec fcmp d29, d30
> 5540f0 b.le 554154
> 554154 bl float_overflow_error ; raises
>
> ; outer multiply -- operand test gone
> 554104 adrp x0, 76c000
> 554108 ldr d29, [x0, #568] ; M_PI
> 55410c fmul d31, d31, d29 ; (r*r) * M_PI
> 554110 fcmp d31, d30
> 554114 b.le 554120
> 554118 mov x0, #0x7ff0000000000000 ; returns +Infinity
> 55411c b 55412c
>
> Control reaches the outer multiply only via the b.le at 5540e4, i.e. only
> when
> r*r <= DBL_MAX, and M_PI is a finite constant. So
> "!isinf(val1) && !isinf(val2)" is true on that path and
> float_overflow_error()
> must be called.
>
> Building from an unmodified 18.3 tree (git tag stamp 62d6c7d) with gcc 14.2
> and
> the same CFLAGS reproduces it, so this is not specific to Debian's
> packaging:
>
> ./configure --without-readline --without-zlib --without-icu \
> CFLAGS="-g -O2 -fno-strict-aliasing -fwrapv
> -fexcess-precision=standard"
> make -C src/backend submake-generated-headers
> make -C src/backend/utils/adt geo_ops.o
> objdump -d geo_ops.o
>
> circle_area then contains 3 fmul but only 1 call to float_overflow_error.
> From
> pristine source the outer check is removed entirely: there is no DBL_MAX
> comparison after the second fmul at all, only the underflow test.
>
>
>
>
>

--
Regards,
Rachitskiy Andrey